Fluxes have been used for many years to prevent the formation of, or to dissolve and facilitate removal of, oxides and other undesirable substances during the brazing of metals. As used herein this specification and the claims below, the term "brazing" generally means a group of welding processes that join solid materials together by heating them to a suitable temperature and using a filler metal having a liquidus above about 450.degree. C. (840.degree. F.) and below the solidus of the base materials being joined.
During the brazing process filler metal is distributed between the closely fitted surfaces of the pieces being joined by capillary attraction. More particularly, with respect to aluminum it is well-known to join two or more pieces of aluminum by disposing an aluminum brazing alloy between or adjacent the faying surfaces of the aluminum pieces being joined and heating the brazing alloy and the faying surfaces to a temperature at which the brazing alloy melts while the aluminum pieces remain unmelted As used herein this specification and the claims below the term "faying surfaces" means the surfaces of the pieces of metal that are in close proximity or contact and are to be joined Upon subsequent cooling, the brazing alloy forms a fillet or joint that bonds the faying surfaces together It is generally preferred that the melting point of the brazing alloy be at least about 30.degree. C. to 40.degree. C. (86.degree. F. to 104.degree. F.) lower than that of the metal of the components being joined Examples of suitable aluminum brazing alloys are alloys which contain aluminum and silicon and have a melting temperature of about 577.degree. C. (1070.degree. F.)
In some applications, one or both of the faying surfaces may be preclad with a layer of aluminum brazing alloy. In other applications, however, the brazing alloy is applied to the faying surfaces in the form of a powdered or particulate mixture carried in a suitable liquid or paste-like vehicle. Although fluxless brazing procedures have been devised, their use in many applications is not preferred. Thus, in many applications it is necessary to employ a flux while brazing aluminum The flux serves to remove the oxide ordinarily present on the faying surfaces, promote flow of molten brazing alloy during the heating step and inhibit further oxide formation. Fluxes are generally reactive when they are in the molten or liquid state. Thus, a flux must be partly or wholly molten at aluminum brazing temperatures in order to allow the flux to properly perform. Aluminum brazing temperatures are generally above about 571.degree. C. (1060.degree. F.)
Wallace et al. U.S. Pat. No. 3,951,328 and Cooke U.S. Pat. No. 3,971,501 both disclose the use of potassium fluoroaluminate complexes in fluxes for brazing aluminum Both Wallace et al. and Cooke define potassium fluoroaluminate complexes to include AlF.sub.3, KAlF.sub.4 and K.sub.3 AlF.sub.6. In both Wallace et al. and Cooke the preferred formulation of the flux comprises an intimate mixture of K.sub.3 AlF.sub.6 and KAlF.sub.4. The assignee of the Wallace et al. and Cooke patents, (i.e., Alcan Research and Development Limited of Montreal, Canada) markets a brazing flux relating to the Wallace et al. and Cooke patents under the registered trademark NOCOLOK. To the best of applicant's knowledge the NOCOLOK brazing flux comprises approximately 90 percent by weight KAlF.sub.4 and about 10 percent by weight K.sub.3 AlF.sub.6. The NOCOLOK brazing flux exhibits a melting temperature of approximately 560.degree. C. (1040.degree. F.). As used herein this specification and the claims below the term "melting temperature" means the temperature at which substantially the entire portion of a compound or a material changes from a solid to a liquid within a period of less than about one (1) minute.
The prior art brazing fluxes have been found to be acceptable for various aluminum brazing applications. However, in some applications the prior art brazing fluxes have left a two salt residue which is not always desirable Also, the prior art brazing fluxes have been known to emit gases which in some applications have required pollution abatement equipment in order to properly handle and/or process the evolving gases.